The Daily Variation of Irregular Disturbances of the Earths's Magnetic Field at Bombay

Abstract

Besides the quiet‐day solar diurnal variation and the variations associated with magnetic storms, the intensity of the Earth's magnetic field at any place is subject to many irregular, short‐period fluctuations. The paper contains an analysis of the diurnal variation of these irregular disturbances during the eleven‐year period 1923–33. As a measure of the disturbance, the departures of the hourly ranges of the horizontal forces from the mean hourly range on the five international quiet days in that particular month were used. In calculating the disturbance, due regard was paid to the sign of the change of the element during the hour. The diurnal variations of the irregular disturbances depend on the season of the year and on the sunspot‐activity. These variations are analyzed and compared with the results of disturbance diurnal variation in higher latitudes. The main conclusions that emerge from the analysis are:(i) The hour of minimum disturbance occurs early in the morning usually between 04h to 05h or 05h to 06h local time and that of the maximum near about noon.(ii) There are two secondary maxima, one at 16h to 18h and another at about 22h to 23h local time.(iii) Analyzing the results according to month, the day maximum is most pronounced in the months April to August. In the winter months, there is a tendency for the variation to approach the European type (as shown by the analysis of the data of Eskdalemuir (magnetic latitude Φ=59°.5) and of Wilhelmshaven (Φ = 54°.5) in which there is one minimum in the morning at about 09h local time and a maximum in the evening at about 22 h.(iv) If days of magnetic character 2 are excluded, the diurnal variations are similar, but the late evening maximum is suppressed, leaving the noon maximum more pronounced.It is generally believed that “in the latitudes between the two auroral zones, that is, up to at least 65° magnetic latitude, Di has a simple diurnal variation with its maximum in the evening; as the auroral zone is approached the hour of maximum gets later, from about 21h at 55° to midnight at 70°. Up to this latitude the form of the daily variation of Di does not vary much either with season or with the general intensity of magnetic disturbance” [Chapman]. The present analysis of the Bombay data shows that in low magnetic latitudes, the main variation is a maximum near noon with a minimum a little before sunrise. The evening maximum of temperate latitudes is superposed on this simple variation.It is suggested that the maxima of disturbance‐variation at Bombay at about noon and in the afternoon are associated with the maxima of ion‐density in the E‐ and F1‐layers and in the F2‐layer, respectively. The late evening maximum is presumably due to fluctuations in F2‐layer caused by electrified particles from the Sun concentrating on the night side of the Earth on account of the deflecting action of its magnetic field.